Colorimetric indicators are a well-known means of detecting the presence of a chemical substance in a particular medium. This type of indicator includes, e.g., pH indicators which exhibit a colour change as the pH of the medium in which it is placed varies.
Such indicators rely on the optical properties of reactive dyes or inks. These dyes can exist in at least two different chemical states, with each form of the dye absorbing light in a particular range of wavelength. When such a reactive dye existing in a first form is exposed to a given substance, it reacts with the substance via a reversible chemical reaction, thereby turning into a second form of the dye. As the second form of the dye absorbs light at a different wavelength, the chemical reaction provides a colour change which is visible by an observer.
The use of colorimetric indicators thus potentially provides an attractive solution to the problem of detecting the presence of some particular chemical substances.
Such substances include gases, such as carbon dioxide, ammonia, and oxygen which have particular significance in, amongst other things, food packaging.
Detection of carbon dioxide has always had significance due to the negative effect of carbon dioxide on health if held in too high concentrations. In medicine, carbon dioxide is one of the key, basic analytes that are routinely monitored in the blood of hospital patients. Capnography is an area in medicine wholly devoted to the monitoring of levels of carbon dioxide in breath. Not only does the presence of carbon dioxide provide important valued medical information, but also its temporal variations in the exhaled breath is used routinely to provide diagnostic information via capnography. In anaesthesiology, one method to ensure the correct placement of the tube carrying the gases to the lungs into the trachea, rather than the oesophagus, is to monitor the level of carbon dioxide (typically 4-5% in exhaled breath).
In the food industry, the use of modified atmosphere packaging (MAP) is well established. MAP packaging involves flushing food with an oxygen-free gas, usually carbon dioxide, and sealing, ready for distribution to the wholesale and/or retail trader. The purpose of MAP packaging is to prevent aerobic spoilage microbe growth, and usually allows food to stay fresh 3-4 times longer. Detection of levels of carbon dioxide in MAP-packaged food is essential to indicate the freshness of the food.
Ammonia (NH3) is a caustic, hazardous gas with a pungent characteristic odour. It is widely used both directly and indirectly in the production of explosives, fertilisers, pharmaceuticals, household cleaning products and as an industrial coolant. Ammonia and other volatile amines also give spoiled fish its ‘off’ taste and smell, as these are produced as fish meat decays. As a result there is a need to monitor ammonia levels not only in industry to monitor for leaks and waste water effluents, but also in the food packing industry, in particular for fish packaging. After fish are caught and killed micro-organisms form on the skin and scales. These are known as specific spoilage organisms (SSO) which produce ammonia and volatile amines including trimethylamine (TMA) and dimethlyamine (DMA) from the amino acids present in the fish. These microbial degradation products are collectively known at total volatile basic nitrogen (TVB-N). By measuring the TVB-N if would be possible to give a measure of how fresh the fish is.
The main cause of most food spoilage is oxygen, because its presence allows a myriad of aerobic food-spoiling micro-organisms to grow and thrive. Oxygen also spoils many foods through enzyme-catalysed reactions, as in the browning of fruit and vegetables, destruction of ascorbic acid and the oxidation of a wide range of flavours. Many oxidative food-spoiling reactions, including lipid oxidation, occur non-enzymically.
A number of colorimetric indicators capable of detecting the presence of particular analytes have been reported in the literature. The reactive dyes employed in such indicators typically have poor thermal stability and/or shelf life, therefore rendering their commercialisation and utilisation in finished articles difficult. For instance, the use of colorimetric CO2 detectors in respiratory medicine has been reported. Examples of such commercial devices include, e.g., Pedi-Cap (Nellcor, Pleasanton, Calif.) and Mini StatCO2 (Mercury Medical, Clearwater, Fla.). However, once removed from their sealed packaging, the average life span under normal atmosphere of such indicators is very short, typically approximately 2 hours for Pedi-Cap, and approximately 24 hours for Mini StatCO2.
In the case of carbon dioxide, solvent based solid, dry carbon dioxide sensors were made possible with the discovery that a phase transfer agent, PTA, is able to extract the anionic form of the colorimetric pH indicator, from the highly polar protic medium into the less polar environment of the polymer/plasticizer. The water associated with the dye is also delivered to the hydrophobic polymer via the PTA. In such plastic thin CO2 film sensors, the equilibrium set up between the dye and carbon dioxide can be represented by the following reaction:
Where α′ is the equilibrium constant associated with the process. Despite the importance of CO2 as an analyte and the significant interest in CO2 indicators, few colour-based CO2 indicators have been commercialised. One of the reasons for this is the poor stability over time of such indicators. Most have shelf lives of less than six months under ambient air conditions due to, amongst other things, a poor thermal stability of the phase transfer agents used and a tendency to react irreversibly with other acid gases such as NO2 and SO2.
Therefore, there is a need in the prior art to develop new chemical indicators, and in particular colorimetric indicators, to provide simple, reliable, and cost effective detection means that exhibit improved storage and thermal stability.
Further, there is a need in the prior art to develop new polymer-based compositions incorporating such indicators, which compositions may be prepared and processed via known polymer processing techniques while maintaining the efficacy and stability of the new indicators.